Electric motor



l- 1945. c. SWIFT 2,333,823

ELECTRIC uo'roa Filed April 28, 1944 UYILH W Patented Aug. 28, 1945UNITED STATES PATENT OFFICE Claims.

This invention relates to an electric motor, and more particularly to aself-starting synchronous motor.

One of the principal features of the present invention is to provide a'small fractional horsepower synchronous motor of the type developingenough torque to drive a clock mechanism or other relatively low powerconsuming device. It is desirable, in motors of this class, to provide astructure which is simple, economical to manufacture, and rugged andreliable in use. It is also desirable to provide a structure in whichthe starting torque as well as the running torque is sufricient tooperate relatively small mechanisms, but which torque is relativelylarge compared to the size of the motor.

It is an object of the present invention to provide a novel synchronousmotor having the above highly desirable characteristics.

It is a further object of the present invention to provide a novelself-starting synchronous motor.

It is a further object of the present invention to provide aself-starting single phase synchronous motor which will always run inthe same direction.

Another object of the present invention is to provide a novel motor ofwhich all of the poles of the rotor are of the same magnetic polarity.

Another and further object of the present invention is to provide anovel electric motor having two stator portions each with an annular setof teeth in proximity to the rotor.

Still another and further object of the present invention is to providea novel magnetic circuit {or an electric motor or other dynamo electricmachine.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization, manner of construction andmethod of operation, together with further objects and advantagesthereoi', can best be understood by reference to the followingdescription taken in connection with the accompanying drawing, in which:

Figure l is a plan view of an electric motor embodying the novelfeatures of the present invention;

Figure 2 is an elevational view of the motor shown in Figure 1;

Figure 3 is a vertical sectional view of the motor as taken along theline I1IIII of Figure 1;

Figure 4 is a. plan view of the motor with the rotor removed; and

Figure 5 is an enlarged fragmentary view of an upper portion of themotor illustrating the relationship of the teeth on the rotor withrespect to the stator structure when the field winding of the motor isdeenergized.

Referring now to the various figures of the drawing, the motor showntherein includes an outer stator ring or sleeve Ill and an inner statorring or sleeve II formed of any suitable ferromagnetic material such asiron or steel, and which are connected together by a base ring I2. Whilemembers III, II and I2 are illustrated as being threaded together thesemembers may be secured together in any other suitable manner such as bymeans of a press flt, or, if desired, they may be formed in one piece.

The outer stator ring I! has an upper flange portion I3 which projectsgenerally radially inwardly and which is provided with a series of teethor pole portions I4.

The inner stator ring or sleeve II is also provided with a series ofteeth or pole portions it which are equal in number to the pole portionsII on the outer stator II! but W with respect thereto, as is clearly sown in gure 4 of the drawing.

The lower end of the inner stator ring or sleeve I I is provided with abase plate I6 which is preferably formed of the same type offerromagnetic material employed for the stator rings III and I I. Thisbase plate I6 may be secured to the inner stator ring II in any suitablemanner, such as by threading, as indicated at H, or may be formedintegral therewith if desired. A strong permanent magnet I8 is mountedon the base I6 with its magnetic axis parallel to the axis of rotationof the rotor 24, and is secured thereto by a bushing I9 having a headportion 20 bearing against the upper end of the permanent magnet It andextending down through a suitable aperture 2| therein. The lower end ofthe bushing I9 is secured to the base plate It in any suitable manner,such as by threadingr as at 22.

The bushing I 8 is arranged to receive and Journal the rotor shaft 23upon which a rotor 24 of ferromagnetic material is rigidly secured. Therotor 24 is provided with an annular series of teeth or pole portions 25which correspond in number to the poles I on the outer stator ring It aswell asvto the pole portions II on the inner stator ring II. The headportion 20 provides a thrust bearing for the rotor 24.

Under certain circumstances it has been found desirable to provide ametal disk 26 mounted on the rotor shaft 23 and seated against the rotor24. The metal disk 26 is made of ferromagnetic material and is free torotate on the shaft 23 with respect to the rotor 24. Such motion isnormally resisted, however, by a relatively weak spring washer 21 whichis held in place by a, collar 28 which is press-fitted on the shaft 23.This provides a greater amount of ferromagnetic material for the fluxthreading the rotor without creating a large mass whose inertia must beovercome in starting the motor. This greatly reduces the requiredstarting torque as compared with that of a solid rotor of equivalentmass.

A field winding 29 is disposed in the annular space between the outerand inner stator rings I and II, as is clearly shown in Figure 3 of thedrawing. A fiber sleeve 30 is also disposed in this annular chamberbetween the outer and inner stator rings In and II, the fiber sleeve 30being seated on the inner stator ring II while the field winding 29 isslipped over the fiber ring 30. This winding 29 is in the form of asolenoid, the solenoid being wound in such a manner that the poles areat the upper and lower ends of the stator structure formed by the outerand inner stator rings I0 and I l. The stator or field winding 29 isenergized through a pair of conductors 3| and 32 which are arranged tobe connected to any suitable source of alternating current, such, forexample, as a 60-cycle, 1l0-volt source.

The stator ring I0 is preferably provided with a plurality ofventilation openings 33 as shown in Figures 2 and 3.

For a reason which will presently be made apparent, each of the rotorteeth 25 has its trailing corner cut back as at 34. The leading edge ofeach rotor tooth is also preferably cut in such a manner as to leanslightly forward in the direction of rotation, as at 35.

The stator teeth I4 on the outer stator ring III are preferably slightlytapered at their ends as at 36.

As shown in Figure 4 of the drawing, the teeth I5 of the inner statorring II are disposed substantially opposite the spaces between theadjacent outer stator ring teeth I4. It has been found that the bestresults may be obtained when the teeth I5 are moved back slightly from aposition directly opposite the spaces between the adjacent outer statorring teeth I4.

Referring now to Figure 3 of the drawing it will be understood that whenthe field coil 29 is deenergized the magnetic circuit of the structureis as follows: The fiux lines produced by the permanent magnet I8 wil1fiow from the upper end of the permanent magnet l8 through the rotor 24(and to some extent through the disk 26) down through the inner statorring II as well as down through the outer stator ring III, through thebase portions I2 and I6 and then back into the lower end of thepermanent magnet III. Since the rotor will tend to move to a positionwhere the greatest fiux concentration lies, it will be apparent that therotor teeth will tend to assume a position partway between th teeth I4of the outer stator ring Ill and the teeth I5 of the inner stator ringII. This position of rest of the rotor 24 is illustrated in Figures 1and 5 of the drawing. The exact position of rest of the rotor 24 may bevaried by varying the proximity of the teeth I5 of the inner stator ringI I with respect to the rotor 24, as compared with the proximity of theteeth I4 of the outer stator ring Hi. It is for the purpose ofillustrating the flexibility of this positioning of the rotor 24 in itnormal position of rest that the various parts of the motor structurehave been shown as being threaded together, but it will, of course, beunderstood that after this positioning of the parts has been located formaximum efliclency that all or substantially all of the threadedconnections may be eliminated.

When the field winding 29 is energized with single phase alternatingcurrent, it will be seen that the upper end of the winding 29 firstrepresents a north pole and then a south pole. For the purposes ofillustrating and discussing the magnetic circuit, let us assume that theupper end of the permanent magnet I8 is a north pole and its lower end asouth pole. When the upper end of the field winding 29 is a north polethe fiux line will extend out the end of the field winding into thefiange I3 and thence diverge, and part of the flux lines will go downthrough the outer stator ring In which provides a. return path thereforand part of the flux lines will return via an air path into the basering I2 and back into the lower end of the solenoid 29. The inner statorring II acts as a core member for the solenoid and the flux lines flowin part therethrough into the teeth 25 and then out into the outerstator ring I0.

At such an instance of time when the upper end of the solenoid is thenorth pole the flux lines produced in the inner stator ring II by thepermanent magnet I9 are in opposition to the fiux lines produced thereinby core 29. The flux lines established in the outer stator ring by boththe permanent magnet I8 and the solenoid 29 are in the same direction. Aa consequence thereof the teeth 25 of the rotor are attracted fashion bysaying that the stator elements I0 and II and the base plate I2 ineffect provide a yoke for the coil 29, and which yoke has relativelyclosely spaced polar portions provided by the teeth I4 and I5. Thus whenthe upper end of the coil 29 is a north pole, the teeth I4 are southpoles and the teeth I5 are north poles, and they hence attract and repelrespectively the north polarized teeth 25 of the rotor 24.

Upon a reversal of the current in the field winding 29, the polarity ofthe teeth l4 and I5 is reversed and hence the polarized rotor teeth 25are attracted to the stator element teeth I5 and repelled by the statorelement teeth I4.

We thus see that continued reversal of the direction of current flowingin the field winding 29 will cause rotation of the rotor 24.

The motor which has been illustrated in the various figures of thedrawing is caused to rotate in the same direction at all times bycutting of! the trailing corners of the rotor teeth 25 and by properlypositioning the teeth I5 of the inner stator ring II with respect to theteeth l4 of the outer stator ring l0.

Referring particularly to Figures 3 and 5, it will be seen that if thedirection of current in the field winding at the instant when theenergizetion circuit is closed is such as to produce a south pole in theteeth I 4, the rotor teeth 25 are attracted to the stator teeth I4 whichlie nearest to the portion of each rotor tooth which is not cut back atthe corner. This would cause rotation of the rotor in the directionindicated by the arrow in Figure 5. It has been found that if the teethI4 are north poles when the energization circuit of the field winding 29is first closed, the rotor teeth will move slightly back in a directionopposite to the normal direction of rotation for the first half cyclebut will immediately thereafter reverse their direction and startrotating in a clockwise direction, as viewed in Figures 1, 4 and 5.

It is believed that this action is due primarily to the fact that thecorner of each rotor tooth 25, which is the trailing corner thereof whenthe rotor is rotating in its desired direction, is cut back. When thecircuit is closed at a time which first produces a north magnetic polein each stator tooth II, the rotor 24 will first move backward a halftooth to a point where the rotor teeth 25 overlie the inner stator ringteeth l5, due to the attraction of the latter. As the field currentreverses direction the stator teeth H on each side of a rotor tooth l5will try to attract the latter. Because of the cutaway corner on therotor tooth the stator tooth lying adjacent the cutaway corner will notexert nearly so great a force of attraction as the stator tooth lyingadjacent the rotor tooth corner which has not been cut away. Thisreverses the direction of initial movement of the rotor and starts it inthe desired forward direction. The rotor gets enough kick in thisdirection that it continues to rotate in a forward direction and is notunstable in its operation. The rotor therefore will always rotate in onedirection only. The operation may sometimes be improved by positioningthe teeth IS with respect to the teeth I so that their radial centerlines do not lie directly opposite the radial center line of the spacesbetween the teeth H, but rather are displaced slightly in acounterclockwise direction with respect thereto.

It has been found that a motor constructed in the manner above describedprovides a relatively large torque for its size and is particularlysuitable for driving clock mechanisms and other small rotatingmechanisms.

While I have shown a particular embodiment of my invention, it will, ofcourse, be understood that I do not wish to be limited thereto, sincemany modifications may be made, and I, therefore, contemplate by theappended claims to cover all such modifications as fall within the truespirit and scope of my invention.

I claim as my invention:

1. A self-starting synchronous motor comprising a rotor having anannular series of radially outwardly projecting teeth uniformly spacedaround the circumference of said rotor, a stator comprising a toothedstator element having its teeth disposed substantially in the plane ofrotation of the teeth of said rotor, a second toothed stator elementhaving its teeth disposed below the plane of rotation of said rotorteeth and in close proximity thereto, said second stator element teethbeing staggered with respect to said first stator element teeth, onecorner of each rotor tooth being cut oil, said out off corne being onthe trailing side of each tooth with respect to the desired direction ofrotation and said cut back corner being so dimensioned that theremaining end portion of each rotor tooth is appreclably less than theend portion of each confronting stator element tooth, means forestablishing an alternating magnetic fiux and for passing a portion ofsaid flux through said first and second stator teeth to alternately andoppositely magnetize the two set of stator teeth, and means forpolarizing said rotor teeth.

2. A self-starting synchronous motor comprising a pair of spacedconcentric paramagnetic stator sh'ells having a. solenoid field windingdisposed therebetween with the axis of the solenoid substantiallycoincidental with the axis of said stator shells, said outer statorshell having an inturned fiange extending over one end of said solenoidand having an annular row of teeth projecting generally radiallyinwardly, said inner stator shell having an annular row of axiallyextending teeth in proximity to said outer stator teeth and 0! the samepitch, a rotor having a plurality of radially outwardly extendingpolarized teeth of the same pitch as said first and second rows of teethand disposed in proximity to said first and second rows of teeth, saidouter stator teeth being staggered with respect to said inner statorteeth, and means for connecting said solenoid field winding to a sourceof alternating current.

3. A self-starting synchronous motor comprising a pair of spacedconcentric paramagnetic stator sh'ells having a. solenoid field windingdisposed therebetween with the axis of the solenoid substantiallycoincidental with the axis of said stato shells, said outer and innerstator shells being connected at one end by an annular member ofparamagnetic material, said outer stator shell having an inturned flangeextending over one end 01' said solenoid and having an annular row ofteeth projecting generally radially inwardly, a roto mounted forrotation about an axis substantially coincidental with the axes of saidstator shells on said solenoid field winding, said rotor being formed ofparamagnetic material and having an annular row of teeth projectinggenerally radially outwardly, said rotor teeth being of the same pitchas said stator teeth, and lying substantially in the same plane as theend of said outer stator shell teeth, said inner stator shell teethbeing positioned and dimensioned to lie immediately below said rotorteeth, said outer stator shell teeth being staggered with respect tosaid inner stator shell teeth, means for connecting aid solenoid fieldwinding to a source of alternating current, a stationary permanentmagnet having a polar element uniformly spaced from the plane ofrotation of said rotor for polarizing said rotor teeth.

4. In a synchronous motor, a ferromagnetic rotor disk having generallyradially extending teeth on the outer peripheral portion thereof, arotor shaft on which said rotor is rigidly mounted, a permanent magnetsurrounding said shaft and having one oi. its magnetic poles disposed inclose proximity to said rotor, a pair 01' concentrically disposedcylindrically shaped ferromagnetic field members, an alternating currentsolenoid field winding disposed between said field members, thelongitudinal axes of said field members and said solenoid beingsubstantially coincidental with the axis of rotation of said rotor, eachof said field members having an annular row of teeth in proximity tosaid rotor teeth and offset with respect to each other, the teeth of thetwo field members being positioned in the flux path of said solenoid insuch a manner that the teeth of one field member are always of oppositepolarity with respect to the teeth of the other field member.

5. In a synchronous motor, a ferromagnetic rotor disk having generallyradially extending teeth on the outer peripheral portion thereof, a.rotor shaft on which said rotor is rigidly mounted, a permanent magnetdisposed in close proximity to said rotor and having one of its magneticpoles disposed adjacent said rotor, a pair of concentrically disposedcylindrically shaped ferromagnetic field members, an alternating currentsolenoid field winding disposed between said field members, thelongitudinal axes of said field members and said solenoid beingsubstantially coincidental with the axis of rotation oi said rotor, each01' said field members having an annular row of teeth in proximity tosaid rotor teeth and staggered with respect to each other,

the teeth of said field members being disposed across the end or saidsolenoid and terminating opposite the ends of said rotor teeth, theteeth of the other of said field members being disposed below the planeof rotation of said rotor and radially inwardly of the teeth of theother field member.

CYRUS SWIFT.

